There are several methods of recording a halftone dot image necessary for producing a print from a gradated original image by means of an image reproduction system, such as a color scanner. A representative one of the methods can be seen in U.S. Pat. No. 4,350,996.
Each of those methods has the following steps. First, according to a coordinate value (U.sub..alpha., V.sub..beta.) of a pixel on a coordinate system (U, V) of a recording film onto which a halftone dot image is to be recorded by a recording beam, a coordinate value (X.sub..alpha..beta., Y.sub..alpha..beta.) of an imaginary screen corresponding to the recording pixel to be recorded on the recording film is computed. Second, a threshold signal corresponding to the coordinate value (X.sub..alpha..beta., Y.sub..alpha..beta.) is read from a data storage means in which a screen pattern corresponding to the imaginary screen is written beforehand at the address (X.sub..alpha..beta., Y.sub..alpha..beta.). Then, the readout threshold signal is compared with the corresponding image signal to consequently form a halftone dot by controlling the recording beam in accordance with the result of the comparison.
Undesirably, each of those primitive methods has a fatal drawback that a periodical interference between the imaginary screen and the locations of corresponding recording pixels causes moire effect on a portion of a unicolor halftone dot image which should be recorded in uniformity by uniform halftone dots. In order to suppress the appearance of moire effect, the method disclosed in U.S. Pat. No. 4,350,996 adopts a superimposition of a random number onto each address of a screen pattern corresponding to the coordinate value (X.sub..alpha..beta., Y.sub..alpha..beta.) of the recording film. This method, however, still has a drawback that a recorded halftone dot carries a complex irregularity on its periphery as shown in FIG. 13(b). The contrasts with recordation by a method which comprises no such superimposition of a random number as shown in FIG. 13(a), whereby impaired inking is presented during subsequent printing processing.